The present invention relates to a rear seal assembly for a shaft which passes through a rear wall of a cooling chamber holding a partially frozen beverage.
One method of producing a partially frozen beverage involves cooling a mixture of syrup, water and CO.sub.2 while continually mixing the liquid. As the temperature approaches the freezing point, the mixture turns to a slush. In order to inhibit solidification of the mixture, the mixture is continually agitated or mixed. This mixing is accomplished by agitator paddles that are connected to a shaft. The shaft generally extends through the axial length of the elongated, cylindrically shaped cooling chamber. The shaft is rotated by a motor that is spaced from the cooling chamber. The chamber must be sealed, except for a beverage input line and a dispensing valve at the front end of the cooling chamber which dispenses the partially frozen beverage, and hence the shaft must be sealed to prohibit contamination of the beverage from the ambient environment or leakage of the beverage from the chamber.
One prior art rear seal assembly includes a plastic rear seal housing and short stub shaft extending therethrough. The stub shaft is keyed to the rear axial end of the main shaft that extends through the interior of the cooling chamber. The short stub shaft is a distinct and separate component as compared with the main shaft. The stub shaft extends axially outboard from the rear wall of the cooling chamber. As used herein, the term "axially outboard" refers to an item that is axially disposed away from a referenced item as compared with an interior, central region in the cooling chamber. "Axially inboard" refers to items closer to the interior, central region of the chamber. The axially outboard end of the stub shaft is rotatably held by a brass bushing that is mounted in an outboard section of the plastic rear seal housing. The axially inboard end of the prior art rear seal assembly terminates in a plastic insert threadably mounted in the plastic rear seal housing. The plastic insert includes a threaded, axially inboard extension that screws into a threaded hole through the rear wall of the cooling chamber. The plastic insert has a passage therethrough that forms a bushing for the axially inboard end of the stub shaft and the axially outboard end of the main shaft. The keyed regions of both shafts are disposed in the bushing of the plastic insert. The plastic rear seal housing is sealed to the outside surface of the rear wall of the cooling chamber by an O-ring.
A commercially available shaft seal assembly is disposed in the interior of the plastic rear seal assembly. Circumferentially surrounding an axially inboard portion of the brass bushing (the other portion of the bushing being mounted in the axially outboard section of the plastic rear seal housing) is a first rubber seal ring. The first seal ring holds a bearing member having an axial end face bearing surface disposed inboard of the seal ring. The axially inboard end face is generally radially aligned with the inboard end of the brass bushing. Another axial end face bearing member is forced against the first axial bearing face by a spring. The second bearing face is part of a unit that rotates with the stub shaft. The second bearing face is held by a metal ring assembly. The ring assembly also retains a rubber seal having a coupling section sealingly mounted on a portion of the stub shaft. The metal ring assembly has two parts. The first part is adapted to move slightly axially with respect to the second part. The spring forces a portion of the metal ring assembly axially outboard thereby forcing the second bearing face against the first. The spring and the first and second bearing faces are all retained within the plastic rear seal assembly that is attached to the outside of the cooling chamber. The stub shaft is keyed to the main shaft at a position generally coplanar to the rear wall of the chamber.
Therefore in the prior art device, the mixture held within the cooling chamber is not completely sealed in the interior of the chamber because some mixture may leak through the keyed rotating shafts and the bushing in the plastic insert. The beverage would then be held inside the plastic rear seal housing. The stub shaft is sealed via the coupling section of the rubber seal held by the metal ring assembly and the two axial end face bearing surfaces.
The plastic outer housing of this prior art rear seal assembly may sometimes leak and break. The beverage trapped in the seal housing sometimes leaks out between the threads of the plastic insert and the plastic seal housing. This leakage occurred with greater frequency when the plastic threads of the insert (either cooperating with the rear chamber wall or the plastic seal housing) became worn due to repeated maintenance of the cooling chamber. Since different materials are used in the prior art device and since the temperature gradient across the end region of the main shaft and stub shaft depends somewhat on the amount of beverage that has leaked between the keyed shafts and the plastic insert bushing, the various components sometimes did not expand or contract to the same degree and hence the seal would sometimes fail. Also, the use of different materials in the seal assembly affected the life of the seal since the components would generally wear at different rates.